A gas turbine, also called a combustion turbine, is a type of internal combustion engine including a rotating compressor coupled to a turbine. Ignition of a fuel in a combustion chamber disposed between the compressor and the turbine creates a high-pressure and high-velocity gas flow. The gas flow is directed to the turbine, causing it to rotate.
The combustion chamber comprises a ring of fuel injectors that direct fuel (typically kerosene, jet fuel, propane or natural gas) into the compressed air stream to ignite the air/fuel mixture. Ignition increases both the temperature and pressure of the air/fuel mixture (also referred to as a working gas).
The working gas expands as it passes through the turbine. The turbine includes rows of stationary guide vanes and rotating turbine blades connected to a turbine shaft. The expanding gas flow is accelerated by the guide vanes and also directed over the rotating turbine blades, causing the blades and the turbine shaft to spin. The spinning shaft both turns the compressor and provides a mechanical output. Energy can be extracted from the turbine in the form of shaft power, compressed air, thrust or any combination of these, for use in powering aircraft, trains, ships and electric generators.
After passing through the turbine section, the working gas flow enters a turbine exhaust case through a nozzle. Inner and outer walls of a conventional exhaust case nozzle include respective inner and outer annular rings, which are typically formed as single piece castings. The exhaust gases pass between the inner and outer rings.
Loads are transferred between the inner and outer walls through a series of radial struts disposed within the exhaust gas flow path. Each strut is encapsulated in an aerodynamic fairing shield. The cross-section of a shield resembles an airplane wing with a rounded leading edge tapering to a thinner trailing edge.
Different rates of thermal expansion between the inner and outer rings cause significant thermal stresses to develop within the strut shields and at the point of connection between the strut shield and the inner and outer rings as hot exhaust gasses flow through this region. These thermal stresses can lead to cracking and fatigue degradation of the strut shields, especially where the shields are joined to the inner and outer annular rings.
One approach to minimize the thermal stresses increases the width of the strut shields; wider strut shields exhibit lower thermal transients, thereby minimizing temperature gradients across the shield. Wider strut shields are also capable of supporting larger loads than thinner shields. However, increasing the strut shield width correspondingly increases the blockage of gas flow in the nozzle gas flow path, which may lead to increased disruption of air flow and a corresponding reduction in gas turbine efficiency.
Thus, there remains a need for further contributions in the area of nozzle technology, especially as related to thermal cracking of the strut shields. The present invention satisfies this need in a novel and non-obvious way.